used in the C calling convention, so it's a bad idea to have reg_FDEFN
be r10.

So, make r10 be the new unboxed register reg_NL7 and replace reg_L2
with reg_FDEFN.  (I was too lazy to renumber all registers, and I
don't think it matters where reg_FDEFN is.)

A simple cross-compile is needed.  Use boot-2005-12-1-ppc.lisp for the
cross-compile script.

compiler/ppc/vm.lisp:
o Make old FDEFN be the new unboxed register NL7.
o Rename L2 to be FDEFN, removing L2 complete.

lisp/ppc-assem.S:
o reg_L2 no longer exists, so don't use it.

lisp/ppc-lispregs.h:
o Make the corresponding changes here that were made in vm.lisp.

bootfiles/19c/boot-2005-12-1-ppc.lisp:
o Simple cross-compile script for this change.